1. Field of the Invention
This invention relates to heat insulators and more particularly to a multilayer heat insulator to make the behavior of the contained gas as small as possible, to prevent thermal conduction and to improve the thermal insulating performance.
2. Description of the Prior Art
In the thermal conduction of a generally used heat insulator (used at a temperature of 100.degree. to 500.degree. C.), a thermal conduction through the component solids and a thermal conduction by the gas contained between the component solids are generated in a complex manner. In order to improve the heat insulating performance of such a heat insulator, it is necessary first to select a substance low in the thermal conductivity for the solids forming the heat insulator and second to utilize air which has a most convenient to the ordinary use and is high heat insulation. For example, in the case of glass wool heat insulator used as a fiber type heat insulator, though the thermal conductivity of the glass fiber itself is generally about 0.89 kcal/mh.degree. C. (at room temperature), by its collection and combination, it will show such remarkably favorable heat insulating performance as a thermal conductivity of about 0.03 kcal/mh.degree. C. (at room temperature) at a bulk density of about 0.03 g/cm.sup.3. Such tendency is also the same in rock-wool, asbestos and the like. In a porous type heat insulator, the heat insulating performance varies with the structure and shape of the porous material.
It is also necessary to prevent radiant heat transfer. Particularly, in the case of a large amount of the contained gas, in case the ambient temperature rises, radiant heat transfer will also tend to increase. In order to make the thermal conduction of the contained gas (for example, air) used as a heat insulator as small as possible, it is necessary to determine the bulk density of the entire structure. It is generally determined by the thermal characteristics (such as the heat insulating performance and temperature characteristic) and using conditions (such as the weight, volume and price) of the component material. Therefore, in order to further improve the heat insulating performance, the behavior of the gas (for example, air) contained between the component solids is an important factor. The conventional fiber type or porous type heat insulator has a great defect in that the contained gas (for example, air) directly becomes a medium of thermal radiation.